Key points
- A mass can remain visible even when lymphoma response criteria describe complete metabolic response.
- Mass size and FDG uptake are different measurements.
- FDG uptake is not specific to lymphoma. Inflammation and healing can also be active on PET.
- One phrase or one number cannot safely choose the next step.
What “residual mass” describes
“Residual” means remaining. “Mass” describes tissue seen on the scan. The phrase does not mean “residual cancer” unless the report or treating team has reached that conclusion using more information.
After treatment, a mediastinal mass may shrink without disappearing. The remaining outline can contain scar-like fibrotic tissue, dead tissue, inflammation or a mix. Imaging can show where tissue remains, but pathology from a biopsy is what directly examines cells when a biopsy is needed and feasible.
CT and PET answer different questions
- CT
- Shows anatomy: location, shape and dimensions. It helps compare how a mass has changed in size.
- FDG PET
- Shows relative uptake of a glucose-like tracer. It adds information about metabolic activity.
- Deauville score
- Compares uptake in the most active area with reference activity in the mediastinum and liver.
The Lugano lymphoma response framework can classify scores 1 to 3 as complete metabolic response at end of treatment even when a mass remains on CT. Read the Deauville score guide for the five scale levels and the limits of reading them alone.
Why residual masses come up in PMBCL
PMBCL usually begins in the mediastinum and can form a large mass before treatment. A visible area may therefore remain after the active lymphoma has responded. At the same time, residual disease cannot be excluded from appearance alone.
PMBCL studies of end-of-treatment PET describe false-positive uptake from treatment-related inflammatory change. They also show why increasing activity, increasing size, new sites and the pattern over time matter to specialist review. This is one reason a single positive-looking scan does not always settle the diagnosis.
Do not assume visible tissue means active lymphoma. Do not assume all remaining tissue or uptake is harmless. Both conclusions require clinical interpretation.
How teams may clarify an uncertain result
Depending on the scan, treatment and access to the mass, a lymphoma team may review the images in a multidisciplinary meeting, compare them closely with baseline, arrange repeat imaging, seek specialist radiology or nuclear-medicine review, or discuss biopsy. Radiotherapy and other treatments are separate decisions, not an automatic consequence of the words “residual mass.”
Each option has benefits, limitations and timing questions. Ask what uncertainty the next test is meant to resolve and how its result would change the plan.
Questions for your medical team
- Is the remaining area smaller, stable or larger than before?
- Is there FDG uptake, and how does it compare with earlier scans?
- What is the Deauville score, if one was assigned?
- How might scan timing or treatment-related inflammation affect the image?
- What does the team think the tissue most likely represents, and why?
- Has the scan been reviewed in a lymphoma multidisciplinary meeting?
- Would another scan or biopsy add useful information in my case?
- What is the next step, when will it happen and what would change the plan?
Medical sources
Claims above link to these sources. Links checked 23 July 2026.